This invention relates generally to a hydraulic chain tensioner having a piston longitudinally movable in a housing and more particularly to a stop mechanism for such a tensioner.
Tensioning devices, such as hydraulic tensioners, are used as control devices for power transmission chains as a chain travels between a plurality of sprockets. In an automotive application, the tension of the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. As a result, it is important to impart and maintain a certain degree of tension to the chain to prevent noise, slippage, or un-meshing of the chain teeth. It is especially important in the case of a chain-driven camshaft in an internal combustion engine to prevent the chain from slipping because the camshaft timing can be misaligned by several degrees, possibly rendering the engine inoperative or causing damage.
In a typical hydraulic tensioning device with a ball-type check valve, fluid flows from a reservoir through a clearance formed between the ball and the seat of a check valve. The hydraulic pressure from an external source, such as an oil pump or the like, flows into a chamber through passages formed in the housing, easily moving the piston outward by the combined efforts of the hydraulic pressure and the spring force.
On the other hand, when the piston tends to move in a reverse direction, the ball is tightly contacted with the ball seat to restrict the flow of fluid from the chamber, thereby preventing retraction of the piston. In this manner, the tensioner achieves a so-called no-return function, i.e., movements are easy in one direction (outward) but difficult in the reverse direction (inward).
A potential problem with hydraulic tensioners of this construction, however, is that they may not always maintain a predetermined tension, especially when an engine is being started or idling at rest with little or no oil pressure. Unless appropriate oil pressure is applied to the chamber, or the chamber is filled with a sufficient amount of oil, the piston moves easily in both directions and loses the no-return function, resulting in noises and vibrations during start-up conditions.
A solution to this potential problem is to provide the tensioner with a rack and ratchet assembly to act as a mechanical no-return device. U.S. Pat. No. 4,874,352 to Suzuki discloses a typical rack and ratchet system in which a rack is formed on the outer surface of the piston and a ratchet is supported in the housing and biased by a spring into meshing engagement with the rack. In this manner, the ratchet ensures that the piston will remain extended outward against tension from the chain or when oil pressure is low. A drawback to such large rack and ratchet systems located on the side of the piston is that the system creates a large hydraulic leak path that limits the effectiveness of tensioner operation.
Referring now to FIG. 7, there is shown a typical tensioner, as known in the prior art, incorporating the cumbersome rack and ratchet assembly, as disclosed in U.S. Pat. No. 4,874,352 to Suzuki. The tensioner 11 includes a housing 40 having a bore 42, which forms a fluid chamber 44 with a hollow piston 46. Preferably, the chamber 44 is a cylindrical bore. The chamber 44 is filled with fluid through passageway 48 from a pressure fluid source (not shown). The fluid source may be an oil pump, oil reservoir, or the like. Check ball 50 is biased toward the ball seat 52 by a ball spring 54 that abuts at one end against a retainer 56 to form a ball-type check valve. The ball-type check valve is provided between the chamber 44 and the passageway 48, and thus the source of fluid pressure, to permit fluid flow into the chamber while blocking fluid flow in the reverse direction. Fluid enters the chamber 44 formed by the bore 42 and hollow piston 46, as described below. The chamber 44 slidably receives the hollow piston 46, preferably cylindrical and having an upper end 58. The upper end 58 of the piston contacts the tensioner face, or shoe member (not shown). The shoe member provides tension along a chain/belt (not shown). A spring 60 contacts the inside of the upper end 58 of the piston 46 to bias the piston in a protruding or outward direction. The piston 46 is fitted with a rack 62 formed on the outer surface thereof. The rack 62 meshes with a ratchet 64 which is rotatably supported in the housing 40 and biased by a spring 66 in a direction opposite to the aforementioned protruding direction.
U.S. Pat. No. 5,346,436 to Hunter et al., which is owned by the assignee of the present application and which is incorporated herein by reference, discloses a rack and ratchet assembly that provides a mechanical no-return function. The rack and ratchet systems disclosed in both U.S. Pat. No. 5,346,436 and U.S. Pat. No. 4,874,352, however, are directed toward primary tensioners which are sized for substantial space requirements not typically available in secondary tensioners. The large size of such primary systems prevents their use on secondary tensioners which lack the substantial amount of space found in other hydraulic tensioners. A need, therefore, exists for a no-return mechanism for use on secondary tensioners.
The stop mechanism of the present invention is a simple and inexpensive apparatus for providing a secondary tensioner with a no-return mechanism. The stop mechanism may also be used on conventional tensioners, the advantages being reduced manufacturing complexity and the replacement by the stop mechanism of five parts found in a typical no-return mechanism. The stop mechanism includes a set of teeth formed on the outside of the tensioner housing and a rack that slips over the end of the piston and engages the teeth on the housing. The stop mechanism of the present invention lacks the ratchet of other no-return mechanisms. The advantage is that the stop mechanism of the present invention is smaller and more compact. This construction allows for use of a no-return mechanism on a secondary tensioner where there are substantial space constraints.